The main problem under consideration is to determine what controls the development and modification of connections between units in excitable systems. We will use three primary preparations that have small numbers of units and whose nerve and muscle cells have been specifically identified. 1) A major preparation will be the giant reticulo-spinal neurons (Muller cells) of the lamprey brain. We have recently identified and characterized the dendritic tree for several these cells. We will study the factors that control the development and form of the dendritic tree in these identified cells by working with cells in which the dendritic branches have been induced to retract and then grow due to axon section. The ultrastructure of these changing identified synaptic junctions will be examined in the electron microscope using intracellular marking techniques. 2) The relationship between cell metabolism and electrical properties of the cell membrane will be examined in central neurons of insects using freeze-fracture electron microscopy. The soma and the axon of identified insect central neurons have different electrical properties. We can change the electrical properties either by injury to the axon or by treatment with colchicine. We will study the ultrastructural changes in the membrane that may be correlated with these differences in electrical responsiveness. 3) We will examine a defined central synaptic complex in the cricket consisting of a single stretch receptor axon that terminates on the identified dendritic tree of a single motor axon innervating flight muscles. A combination of afferent terminal degeneration and intracellular staining of the dendritic tree will be employed to determine the 3-dimensional relationship of this model dendritic complex. The electrophysiology of this system will be studied by intracellular recording from the identified dendritic tree. Changes in this defined synaptic complex will then be studied by removing the soma of the postsynaptic neuron and also by cutting the presynaptic axon and observing degeneration and subsequent regeneration of the identified synaptic complex.